Railroad hopper cars are commonly used for transporting dry materials in bulk. Such cars typically include one or more large manhole size hatches on the top of the car for loading material into the car, with several smaller spouts on the bottom for unloading the material. Such cars are typically unloaded by applying a vacuum conveying line to an outlet gate positioned at the bottom of each car compartment. As can be appreciated, as material is drawn out from the car using the vacuum technique, it is essential that there be some opening in the top of the car to prevent a negative pressure that could cause the car or compartment to implode.
A recent development in hatch covers includes a vented hatch cover such as that shown in U.S. Pat. No. 4,819,830 to Salco Products, Inc., the teachings of which patent are hereby incorporated by reference. Such vented hatch covers avoid the need for a worker to climb to the top of the car to open one or more hatch covers or other air entry openings to provide the air inlet for pressure compensation.
It is of course important to protect the contents of the car from contamination during transportation and unloading. Although unvented hatch covers do not generally require any additional contamination protection during transportation, if they must be opened during the unloading process, contamination becomes a problem. For this reason, hatch seals which function as filters that permit air to be drawn into the hatch but filter out contaminants have been used. Such filters are shown in U.S. Pat. No. 4,902,173 to Hendee Enterprises, Inc., the teachings of which are hereby incorporated by reference. To protect the car contents from moisture during transportation, thin plastic membrane seals have also been used over the hatch openings, as mentioned in U.S. Pat. No. 4,902,173.
The use of plastic membrane seals which must be removed before the car can be unloaded takes away the labor and safety advantage of using a vented hatch cover because a worker must go atop the car to remove the seal from the hatch before the car can be unloaded. If the seal is not removed, it could act as an air infiltration barrier, which could cause the imploding problem or could result in the filter being drawn into the compartment and itself becoming a contaminant of the material.
Thus it would be desirable to have a moisture barrier that can be used with a vented hatch that provides a moisture barrier function until the car is ready to be unloaded but does not have to be removed before unloading the car. It would further be desirable to have a single seal which provides both a moisture barrier and a particulate filter which could be placed over the hatch after the car is loaded but does not have to be removed from the hatch before unloading the material from the hopper car.